its a dynasty 3.3. why doesn't the serpentine belt stay on on 1990 Dodge D250

this last summer i bought my granddaughter a 1990 dodge dynasty. we've put on three serpentine belts on and they keep coming off. can someone help me? from lorraine.
crying in drayton nd

1 answer 1 comment
TRY A NEW BELT TENSIONER
i'm sorry, i guess i forgot to say we did that also. but thanks for a good answer.lorraine